Method for producing osmotically and mechanically stable gel-like anion exchangers

ABSTRACT

The present invention relates to a process for preparing gel-like anion exchangers by aminating crosslinked gel-like styrene divinybenzene bead polymers that contain halogenoalkyl groups in the presence of an inorganic salt and in the absence of organic solvents.

The invention relates to a process for preparing basic anion exchangerswith improved mechanical and osmotic stability.

Nowadays there is a wide variety of interesting applications for anionexchangers. For example, they are used in treating drinking water, inpreparing ultrahigh-purity water (needed for the computer industry inmicrochip production), for purifying and decolourizing sugar solutionsand for removing heavy metal complexes from solutions.

For all of these applications it is desirable for the anion exchangers,which are in the form of beads, to retain their form and not to fragmentor lose their structure, partially or completely, during use. If thishappens, these polymer fragments can pass into the actual solutions tobe purified and contaminate them during the purification process. Thepresence of damaged bead polymers is moreover detrimental to thefunctioning of the very anion exchangers used in column processes.Fragments cause increased pressure loss in the column system, thusreducing the throughput of liquid to be purified through the column.

Various factors affect the physical stability of the anion exchangers.These include the conditions of bead polymer preparation, and also theconditions of amination of the bead polymers prepared, which arearomatic, crosslinked copolymers containing haloalkyl groups.

The object of the invention was therefore to provide a process forpreparing anion exchangers with improved stability. Surprisingly, it hasnow been found that this object is achieved by aminating the gel-likechloromethylate in an aqueous solution comprising small amounts of aninorganic salt.

The invention therefore provides a process for preparing gel-like anionexchangers by aminating crosslinked gel-like styrene divinylbenzene beadpolymers which contain halogenoalkyl groups.

The base polymer is a crosslinked polymer of monoethylenicallyunsaturated monomers composed predominantly of at least one compoundfrom the series styrene, vinyltoluene, ethylstyrene, α-methylstyrene andring-halogenated derivatives of these, such as chlorostyrene.

In recent times, ion exchangers with a very uniform particle size(termed “mono-disperse” below) have become increasingly important, sincethe more advantageous hydrodynamic properties of an exchanger bed madeof monodisperse ion exchangers can achieve economic advantages in manyapplications. One of the ways of preparing monodisperse ion exchangersis that known as the seed-feed process, in which a monodispersenonfunctionalized polymer (“seed”) is steeped in monomer, and this isthen polymerized. Examples of patent specifications which describeseed-feed processes are EP-0 098 130, EP-0 101 943, EP-0 418 603, EP-0448 391, EP-0 062 088, U.S. Pat. No. 4,419,245.

Another way of preparing monodisperse ion exchangers is to prepare theunderlying monodisperse bead polymers by a process in which the uniformmonomer droplets are formed by subjecting monomers to vibration while inlaminar flow, and then carrying out polymerization, U.S. Pat. No.4,444,961, EP-0 046 535.

A process carried out industrially for preparing anion exchangersstarting from bead polymers based on styrene divinylbenzene (DVB)proceeds by first functionalizing (chloromethylating) the aromatic ringsystems present in the bead polymers, using chloromethyl groups, andthen reacting these with amines.

In the chloromethylation the crosslinked bead polymer based onstyrene/divinyl-benzene (DVB) reacts with monochlorodimethyl ether usingFe₂O₃, FeCl₃, zinc chloride, tin(IV) chloride, aluminium chloride orother Friedel-Crafts compounds as catalysts, releasing methanol andother components—see EP-0 327 255.

The preparation of monochlorodimethyl ether is usually accompanied bythe production of carcinogenic bischlorodimethyl ether. There arevarious versions of the process for preparing the monochlorodimethylether and for its reaction with bead polymers based on styrenedivinylbenzene (DVB)—see EP-0 327 255, U.S. Pat. No. 4,225,677, U.S.Pat. No. 4,207,398, U.S. Pat. No. 5,523,327, DD-250 128, U.S. Pat. No.4,207,398.

An excess of the chloromethylating agent is usually used, since thisacts not only as an agent but also as a medium for steeping the beadpolymer—see EP-0 776 911.

After the chloromethylation there are various ways of separating theremaining reaction medium, which in particular comprisesmonochlorodimethyl ether, from the chloromethylated bead polymer and forworking up the chloromethylate.

JP-A-7-188 333 removes the remaining monochlorodimethyl ether by solventextraction after the chloromethylation.

EP-0-776 911 meters in aqueous hydrochloric acid after thechloromethylation, heats the mixture to 110° C. and distils unreactedmonochlorodimethyl ether. The chloro-methylate is centrifuged andprecipitates as a moist product.

EP-0-327 255 meters in methanol and formaldehyde, and, if desired, alsomethylal, after the chloromethylation. The mixture is stirred, andhydrochloric acid is metered in after about 1 hour. Themonochlorodimethyl ether, both that already present and that newlyformed, is distilled off. The chloromethylate is, if desired, washedwith methylal, then with water, and then neutralized with aqueous sodiumhydroxide.

EP-0 481 603 adds methanol after the chloromethylation in order to breakdown remaining monochlorodimethyl ether. The gel-like chloromethylate isthen washed with methanol to remove by-products.

DD-250 129 separates off the chloromethylate after the chloromethylationvia a frit, and then washes the product with methanol.

There are various ways of reacting the chloromethylate obtained to giveanion exchangers, using various amines.

In industry use is often made of anion exchangers having tertiary—orquaternary ammonium groups. For example, use is commonly made of anionexchangers having trimethylamine and/or dimethyl-or hydroxyethylammoniumgroups.

EP-0 776 911 describes the amination of an aromatic, crosslinkedcopolymer containing haloalkyl groups. The actual copolymer is a porousbead polymer, prepared by suspension polymerization. Examples 1 to 4describe the amination of porous chloromethylates in aqueous sodiumchloride solutions with addition of toluene. The chloromethylate isreacted with an amine in the presence of at least 100 parts by weight ofwater per 100 parts by weight of chloromethylate, and at least 5 partsby weight of a water-soluble inorganic salt per 100 parts by weight ofwater, with addition of toluene. The amine used comprisestrimethylamine, and the inorganic salt used comprises sodium chloride inthe presence of organic solvents, such as benzene, toluene, xylene ordichloroethane. The temperature for the amination is 50° C. Theresistance of the resultant anion exchanger beads to pressure wasmeasured. The presence of at least 5% by weight of sodium chloride inthe water during the amination considerably increases the resistance ofthe anion exchangers to pressure when comparison is made with theproduct prepared without sodium chloride.

U.S. Pat. No. 5,182,026 describes the amination of an aromatic,crosslinked copolymer containing haloalkyl groups. The actual copolymeris a porous bead polymer prepared by suspension polymerization. Examples1 to 3 and A to C describe the amination of porous chloromethylates. Theamination is carried out in two steps. The first reagents used areprimary or secondary amines, resulting in reaction of from 15 to 95% ofthe haloalkyl groups. The partly aminated resin is then reacted withtertiary amines, such as trimethylamine or triethylamine, to givestrongly basic anion exchangers. The first amination is carried out inwater with addition of from 100 to 280 g of sodium chloride, and also ofa base, such as NaOH, at temperatures from 60 to 100° C. The copolymerused may also comprise a gel-like polymer prepared by the seed process.

EP-0 481 603 describes the amination of gel-like copolymer beadsprepared by a seed process. The bead polymers have core-shellmorphology. This means that the poly-meric structure of the beads varieswith the distance from the bead centres. During the haloalkylation,post-crosslinking takes place via introduction of methylene bridges. Thepost-crosslinked, chloromethylated gel-like bead polymer is aminated insteeping agents, such as water, or in a mixture of water with organicsubstances, such as methanol, methylal or methylene chloride, usingaliphatic, secondary amines at temperatures between 60 and 100° C. Abase, such as sodium hydroxide, is moreover added. Sodium chloride isnot used. The secondary amines used in at least stoichiometric amountscomprise dimethylamine, diethylamine or dipropylamine. Thefunctionalization introduces predominantly weakly basic groups into theresin. Post-crosslinking moreover produces amine bridges.

EP-0 327 255 describes the amination of a chloromethylated gel-likecopolymer based on styrene and divinylbenzene. An aqueous slurry of thechloromethylate is mixed with aqueous sodium hydroxide. Gaseoustrimethylamine is distilled into the autoclave. The mixture is stirred,first at 15° C. and then for 6 hours at 28° C.

EP-0 277 795 describes the amination of a chloromethylated styrenedivinylbenzene copolymer. The copolymer contains from 0.5 to 2% byweight of divinylbenzene. The water-steeped chloromethylate is mixed inwater with aqueous trimethylamine solution and stirred within the rangebetween 5 and 35° C.

Another example impacts chloromethylate prepared from 100 g ofcopolymer. 74 g of sodium chloride, aqueous sodium hydroxide and aqueoustrimethylamine solution are metered in. Distillation is then carried outbeginning at temperatures between 0 and 20° C., to remove thetrimethylamine. Fragmentation of all of the beads has occurred.

In another example the procedure is as above, but the beads are slurriedin ethyl acetate. After the amination the anion exchangers are stable.

U.S. Pat. No. 4,419,245 chloromethylates gel-like copolymers based onstyrene divinyl-benzene and prepared by the seed-feed process. One moleof chloromethylate is taken up in water and rendered basic (pH>13) usingaqueous sodium hydroxide. 1.5 mol of anhydrous trimethylamine aremetered in. The suspension is held for from 8 to 12 hours at roomtemperature, then heated to 100° C. to remove remaining amounts ofamine.

In DE-19 634 393 methanol-moistened chloromethylate is reacted in 2.5%strength by weight aqueous sodium chloride solution withdimethylaminoethanol. Nothing is said about the stability of the resin.Since the chloromethylate is methanol-moistened, the amination iscarried out in a water/methanol mixture.

The publications mentioned show that the reaction conditions for theamination substantially affect the stability of the resultant anionexchanger.

There is a need to prepare stable anion exchangers by a process whichdispenses with the use of organic solvents and keeps the amount ofinorganic salt—such as sodium chloride—as low as possible.

Specifically, residues of the organic solvents always remain within theresin and, when the resins are used as anion exchangers, contaminate thewater to be purified. This requires complicated process technologymoreover to separate off the organic solvent present in the reactionmixture after the amination. The amount of inorganic salt should be aslow as possible, for environmental reasons and for cost reasons.

The present invention describes the preparation of an anion exchanger byreacting an aromatic, crosslinked copolymer containing haloalkyl groupswith an amine. The amination is carried out in the presence of anaqueous solution of an inorganic salt. Any inorganic salt may be used,as long as it is water-soluble. This includes water-soluble halides,carbonates and sulphates of alkali metals, such as sodium and potassium,and of alkaline earth metals, such as magnesium and calcium. It ispreferable to use sodium chloride.

The amount of the inorganic salt used is less than 5% by weight, basedon the total amount of water. It is preferable to use from 1 to 4.5% byweight, particularly preferably from 1.5 to 4% by weight, of inorganicsalt, based on the total amount of water. The total amount of water isthat resulting from adding the amount of water added to the amount ofwater introduced in the aqueous amine solution.

The moisture content of the resin is not taken into account at thisjuncture.

Water serves various purposes during the amination. On the one hand itis a stirring medium and on the other it is a reaction medium or elsesolvent for the amine in the present invention.

During the amination the resin continuously takes up water and in doingso swells. A minimum amount of water is therefore necessary so that themixture can at least be stirred. There is no particular upper limit forwater. However, an upper limiting range results from the fact that usinga very large amount of water gives a small concentration of the aminefor a given molar ratio of amine to chlorine (in the chloromethylate).The space-time yield also becomes lower if the dilution used is veryhigh. In addition, the amount of inorganic salt which has to be used fora given salt concentration increases.

At least 1.5 grams, preferably 3 grams, of water should be used per gramof bead polymer containing halogenoalkyl groups. It is preferable forchloromethylate to be used as halogenoalkyl group.

It is not necessary to add any organic solvent.

The aminating reagent used preferably comprises trimethylamine,triethylamine, tripropylamine, tributylamine.

If, for example, use is made of bead polymers containing chloromethylategroups, the amount of amine used is that required for the completereaction of the chloromethylate. This must be at least 1 mol of amine,based on 1 mol of chlorine in the chloromethylate. It is preferable touse 1.15 mol of amine per mole of chlorine.

The temperature at which the amination is carried out may be in therange between room temperature and 160° C. Preference is given tooperating at temperatures of between 70 and 120° C., and particularpreference to the range between 70 and 110° C.

After the amination, the liquid is separated off from the aminated beadpolymer. For purification, the bead polymer is taken up using aqueous,dilute hydrochloric acid and stirred for from 1 to 6 hours attemperatures between 40 and 90° C.

The concentration of the hydrochloric acid may be in the range from 1 to15% by weight, preferably from 2 to 5% by weight.

Measurement of Resin Stability

Determination of the number of perfect beads in original condition: 100beads are studied under the microscope. The number of beads with cracksor fragmentation is determined. The number of perfect beads is given bythe difference between 100 and the number of damaged beads.

Determination of the number of perfect beads by the swollen-stabilitytest: 25 ml of anion exchanger are placed into a column into thechloride form. 4% strength by weight aqueous sodium hydroxide,high-purity water, 6% strength by weight hydrochloric acid anddemineralized water are applied in succession, the aqueous sodiumhydroxide and the hydrochloric acid flowing downwards through the resinand the high-purity water being pumped upwards through the resin. Timecycles produced by a control device are used for the elution. After 20hours, 20 operating cycles have been completed. Once the operatingcycles have ended, 100 beads are counted out from the resin sample. Thenumber of beads in 100 damaged by cracking or fragmentation, i.e.imperfect, is determined.

Preparation of the Copolymer

The copolymer is prepared by the seed-feed process described in theapplications EP-0 098 130, EP-0 101 943, EP-0 481 603, EP-0 448 391,EP-0 062 088,

U.S. Pat. No. 4,419,245, and also in the example described inapplications U.S. Pat. No. 4,444,961, EP-0 046 535.

The copolymer may moreover also be prepared by the processes describedin U.S. Pat. No. 2,788,330 and U.S. Pat. No. 3,509,078, and also byprocesses in Vinyl and Related Polymers, C. Schildknecht, Wiley, 1952,pp.68-75.

EXAMPLE 1 Preparation of a Bead Polymer

3.59 g of boric acid and 0.99 g of sodium hydroxide are dissolved in1160 ml of deionized water in a 4 l glass reactor. 190.5 g of amicroencapsulated spherical styrene bead polymer with 0.5%divinylbenzene content are dispersed into this solution as seed (averageparticle size 215 μm). The microcapsule is composed of aformaldehyde-hardened complex coacervate made of gelatine and of acopolymer of acrylamide and acrylic acid. A mixture from 845 g ofstyrene, 48 g of acrylonitrile, 116 g of divinylbenzene (80%,commercially available isomer mixture of divinylbenzene andethylbenzene) and 8 g of dibenzoyl peroxide (75%, commercially availablewater-moistened grade) is then metered in to the mixture over a periodof 30 min, followed by stirring for 1 h. After this period 60 ml of a 2%strength by weight methylhydroxypropyl cellulose solution are added. Themixture is polymerized to completion by increasing the temperature,cooled, washed via a 32 μm screen and dried. This gives 1150 g of aregularly shaped bead polymer. The particle size distribution of thebead polymer (average particle size 400 μm) reflects precisely theparticle size distribution of the seed.

EXAMPLE 2 Chloromethylation of a Bead Polymer

A mixture made from 1600 g of monochlorodimethyl ether, 165 g ofmethylal and 5 g of iron(III) chloride is charged to a 3 l sulphonatingbeaker, and 300 g of bead polymer from Example 1 then added. The beadpolymer is steeped for 30 min at room temperature and heated to refluxtemperature (55° C.) over a period of 3 h. Stirring is then continuedfor a further 1.75 h at reflux. During the reaction time about 275 g ofhydrochloric acid and low-boiling organic compounds are driven off. Thedark brown reaction suspension is then filtered off, the chloromethylatewashed thoroughly with a mixture of methylal and methanol, then withmethanol, then with high-purity water. This gives 680 g ofwater-moistened, chloromethylated bead polymer.

Chlorine content: 18.7%.

100 ml of chloromethylate moist from filtration weigh 65.9 g. Thisamount comprises 12.45 g of chlorine, corresponding to 0.351 mol.

EXAMPLE 3 Reaction of the Chloromethylated Bead Polymer From Example 2With Trimethylamine

1179ml of high-purity water are charged to the autoclave. 136.4 g ofsodium chloride are metered into the autoclave. 800 ml ofchloromethylated bead polymer moist (with water) from filtration(corresponding to 527.3 g of dry chloromethylated bead polymer with achlorine content of 18.7% by weight) and 401.3 g of 45% strength byweight aqueous trimethylamine solution are added to the mixture.

The suspension is heated to 120° C. over a period of 1 h and stirred fora further 3 hours at 120° C. After cooling to room temperature themother liquor is filtered off with suction, the resin is mixed with 800ml of high-purity water and stirred for 30 min at room temperature. Thewater is distilled off and replaced by 2000 ml of 3% strength by weightaqueous hydrochloric acid. The suspension is stirred for 4 h at 70° C.After cooling, the liquid is distilled off, the resin is taken up usingwater and eluted upwards in a column with 6 bed volumes of high-puritywater. Yield: 2170 ml of resin after agitation.

EXAMPLES 4 To 9

The procedure was the same as that in Example 3, except that the amountof sodium chloride was varied.

EXAMPLE 10

The procedure was the same as that in Example 3, except that no sodiumchloride was used.

Table 1 gives data on the amounts used for the various examples.

TABLE 1 Example 3 4 5 6 7 10 Water in grams 1179 1179 1179 1179 11791179 NaCl in grams 136.4 102.5 75.3 49.1 24.1 0 45% strength 401.3 401.3401.3 401.3 401.3 401.3 by weight trimethylamine solution (TMA) in gramsWater in the 220.7 220.7 220.7 220.7 220.7 220.7 TMA solution in gramsTotal amount 1399.7 1399.7 1399.7 1399.7 1399.7 1399.7 of water % byweight of 9.75 7.32 5.38 3.51 1.72 0 sodium chloride, based on theamount of water

Table 2 gives results from the testing of the final products.

TABLE 2 Example 3 4 5 6 7 10 Number of 98 99 98 98 96 97 perfect beadsin original con- dition per 100 Number of 97 97 97 98 92 87 perfectbeads in the swelling- stability test per 100 Exchange 1.331 1.307 1.2881.324 1.312 capacity in mol/l of resin Residual 0.33 0.15 0.071 0.0680.41 0.05 chlorine con- tent in % by weight

The data in Tables 1 and 2 show that use of a 1.72% strength by weightsodium chloride solution (Example 7) compared with a purely aqueoussolution (Example 10) increases the swelling stability of the beads from87 to 92 perfect beads. Use of a 3.51% strength by weight sodiumchloride solution (Example 6) increases the stability to 98% of perfectbeads. Further increase of the sodium chloride concentration does notgive any increase in stability. In contrast, the stability valuedecreases by one point.

What is claimed is:
 1. A process for preparing osmotically andmechanically stable gel-like monodisperse anion exchangers comprisingaminating a crosslinked, gel-like monodisperse styrene divinylbenzenebead polymer having halogenoalkyl groups by reaction withtrimethylamine, triethylamine, tripropylamine, or tributylamine in anaqueous solution of an inorganic salt, wherein the inorganic salt is awater-soluble halide, carbonate, or sulphate of an alkali metal oralkaline earth metal used in an amount that is less than 5% by weightbased on the total amount of water.
 2. A process according to claim 1wherein the amination is carried out in the absence of organic solvents.3. A process according to claim 1 wherein the amination is carried outat a temperature between room temperature and 160° C.
 4. A processaccording to claim 1 wherein 1.5 grams of water are used per gram of thedry bead polymer.
 5. A process according to claim 1 wherein thehalogenoalkyl groups are chloromethyl groups.
 6. A process according toclaim 5 wherein complete amination is carried out using at least 1 molof an amine per 1 mol of chlorine in the chloromethyl groups.
 7. Aprocess according to claim 1 wherein 1 to 4.5% by weight of theinorganic salt is used.
 8. A process according to claim 1 wherein 1.5 to4% by weight of the inorganic salt is used.